The present invention relates to a process for applying a corrosion resistant, chromate-free conversion coating to a product formed from magnesium or a magnesium alloy and to a coating solution used in the process.
Magnesium alloys are light and strong, but very vulnerable to corrosion due to the reactive nature of magnesium. Magnesium alloys are protected from corrosion in all practical applications. A commonly used, low cost, corrosion resistant treatment for magnesium alloys is a dichromate based conversion coating. While dichromate based conversion coatings provide good corrosion protection, they are based on a chemical compound (hexavalent chromium) that has many occupational exposure risks. A non-chromated, corrosion resistant magnesium conversion coating is required to meet industry demands.
Another treatment for protecting magnesium or magnesium alloy products is shown in U.S. Pat. No. 5,683,522 to Joesten, which is hereby incorporated by reference herein. In this treatment, a paint adherent and corrosion resistant coating of magnesium phosphate and magnesium fluoride is applied to a product formed from a magnesium alloy. The process for applying the coating involves immersing the magnesium alloy product in a solution having phosphate and fluoride ions. This treatment while providing a barrier film and very good paint adhesion, does not include electrochemically active ingredients to suppress corrosion.